(1) Field of the Invention
This invention is in the field of microprogrammed central processors, and more particularly relates to improvements which reduce the number of control words stored in the basic operations control store needed to execute required basic operations in a unit of a large scale pipelined central processor that is implemented on a single very large scale integrated circuit (VLSI) chip.
(2) Description of Related Art
Microprogrammed central processing units (CPU)s are well known. They typically include one or more read only control stores, or ROMs, in which are stored control words. Each control word includes a plurality of control fields, with the bits of each control field being the control signals that cause the circuits of a CPU to execute an instruction.
Depending on the complexity of the central processor, the number of bits in a control word can be quite large, exceeding 100 in some cases. In large scale computer systems it is a common requirement that the system be upward compatible, which means that it be able to execute any application program that was written to run on any predecessor system without the necessity to modify or rewrite such earlier application programs. This results in a requirement that the CPUs of the latest version of the system be able to execute a large number of instructions approaching one thousand in at least one example.
The instruction to be executed is written into an instruction register of the CPU and the operation code, or op. code, control field of the instruction is typically the address in a read only control store of a control word that will cause the CPU to execute the instruction. Implementation of such a CPU when pipelined complicates the circuits of the CPU since not all the fields of a given control word required to execute a given instruction may be used during one time slot, but rather over several time slots.
With the advent of VLSI chips, it is common practice to place all of the circuit elements including control store ROMs of a significant portion of a CPU of a large scale computer system such as an address and execution (AX) unit, a floating point (FP) unit, or a decimal numeric (DN) unit, for example, on a single VLSI chip. Notwithstanding the large number of circuit elements that can be placed on a single VLSI chip, the space available is limited and this limitation applies to control stores which in turn limits the size of the control stores.